Equation without a label.
\begin{equation*}
\begin{split}E = mc^2\end{split}
\end{equation*}
Equation with label.
\begin{equation}\label{equation:equations:test}
\begin{split}E = hv\end{split}
\end{equation}
Second equation without label.
\begin{equation*}
\begin{split}c^2 = a^2 + b^2\end{split}
\end{equation*}
Equation with label \eqref{equation:equations:test} is important.
